1. Field of the Invention
The present invention relates to an optical isolator and particularly to an optical isolator capable of self-alignment.
2. Description of Related Art
Optical isolators are key elements in optic communication systems in which optical signals are produced by lasers, such as semiconductor lasers. In a projected information highway system, for example, semiconductor lasers are employed in transmitters to produce forward directed optical signals. As is well known, the optical signals produced by such lasers may carry different kinds of information, such as digital, analog, or combined format.
The semiconductor lasers are susceptible to light signal reflections and adverse effects, such as optical wavelength jitter, laser output intensity noise, and uncontrolled optical power modulations are caused.
Uncontrolled optical power modulation leads to a non-linear laser transfer function which represents the relationship between laser drive current and output optical power. This causes an infidelity representing the electrical signal (RF) with a distorted optical signal, which may cause errors in the operation of connected circuitry, such as decision making circuitry to misidentify an intended one state for a zero state. In the case of analog systems, such as multi-channel television (TV) systems, non-linearity in the laser transfer function may cause interference between channels.
Laser output intensity noise is induced with laser operation subject to undesired optical return, which degrades the TV signal to noise ratio and as a result reduces picture quality. An optical wavelength jitter transmitting in an optical fiber further produces TV signal distortion due to signal dispersion, as the optical signals propagate along the path of an optical fiber.
In the case of digital systems, information is carried by bit symbols at a rate of 2.4 billion bits per second (i.e. 2.4 Gb/s) or higher. At such rates, the bit to bit spacing becomes progressively more limited. As optical signal bits progress along the length of an optical fiber, the bits are subject to dispersion, which reduces the signal level of the bits. The reduced signal level results in an increased bit error rate. Signal dispersion causes a spreading of the signal and results in bit overlap. The overlapping of bits in turn causes a high bit error rate and reduces fidelity in the transmission of information.
Accordingly, it is desirable to block the reverse or return transmission of optical signals back to a laser transmitter while providing low attenuation at the forward direction.
Further, it is desirable to reduce undesired levels of reflected optical power in optical systems incorporated in communication systems and in the information super highway.
An optical isolator is used with fiber optic amplifiers in optical systems to prevent oscillation due to reflection and to prevent injection of spontaneous optical emissions to the laser transmitter originally producing the optical signals. Optical interference noise effects such as the spontaneous emissions can occur at a reflection level of one part per million (i.e., below 60 dB) of light. The interference noise will increase transmission noise of the fiber optic transmission system resulting in reducing signal to noise ratio and signal distortion.
An optical isolator consists of a number of elements, which typically include a first GRIN lens, a first birefringent crystal wedge, an optical rotator, a second birefringent crystal wedge and a second GRIN lens The first GRIN lens receives and converges rays emitted from an input optical fiber into parallel rays. The first birefringent crystal wedge split the parallel rays into a first ray polarized along the optical axis and a second ray polarized perpendicularly to the optical axis. The second birefringent crystal wedge recombines the first ray and the second ray The second GRIN lens focuses the recombined rays into an output optical fiber The optical rotator is mounted between the first and the second birefringent crystal wedges for rotating the first ray and the second ray 45?at the same direction. The rays reflected from the output optical fiber will be diverged by the isolator and cannot be focused into the input optical fiber.
The elements of the optical isolator must have precise relative orientation with respect to each other in order to achieve the desired performance. This complicates the assembling process of the optical isolator. Thus a variety of methods have been developed for realizing efficient assemble of the optical isolators.
U.S. Pat. No. 5,446,813 discloses a conventional optical isolator capable to prevent ray reflected from output fiber from focusing into input fiber. As shown in FIG. 1 of the attached drawings, the optical isolator shown in U.S. Pat. No. 5,446,813, designated with reference numeral 10 comprises two standard collimators 20, 40 and an isolation assembly 30. Each collimator 20, 40 comprises a capillary 22 retaining an end of an optic fiber 21, 41, a GRIN lens 23 and a glass sleeve 24, wherein the capillary 22 and the GRIN lens 23 are respectively received in the glass sleeve 24. To hold and protect the collimator 20, the glass sleeve 24 is received in a copper sleeve 25. The isolation assembly 30 includes first and second birefringent crystal wedges 31, 32, an optical rotator 33 mounted between the crystal wedges 31, 32 and a magnetic ring 34 mounted between the first and the second birefringent crystal wedges 31 and 32 The isolation assembly 30 is retained in a copper sleeve 36 for protection.
To assemble, the copper sleeves 25, 36 are all fit into a stainless steel sleeve 55 and a bonding agent, such as epoxy, is applied between the copper sleeves 25, 36 and the stainless steel sleeve 55. A bonding agent is then cured to secure the sleeves 25, 36, 55 together. Curing the bonding agent by heat may cause relative displacement between the copper sleeves 25, 36 and the steel sleeve 55 and consequently poor alignment between the elements of the optical isolator.
Furthermore, the conventional optical isolator has a complicated structure and is thus difficult to manufacture.
It is desired to provide an improved structure of optical isolator for alleviating the above problems.
Accordingly, an object of the present invention is to provide an optical isolator having a simple structure for allowing easy and efficient assembly.
Another object of the present invention is to provide an optical isolator having excellent alignment result.
In accordance with the present invention, an optical isolator is provided, comprising a first collimator, a second collimator and an isolation assembly arranged between the collimators. The first collimator comprises a first capillary retaining an input fiber and a first GRIN lens received and retained in a first glass sleeve. The isolation assembly comprises two birefringent crystal wedges with an optical rotator interposed between the birefringent crystal wedges. A magnetic ring receives and retains the birefringent crystal wedges and the optical rotator together. The collimator comprises a second capillary retaining an output fiber and a second GRIN lens which are received and retained in a second glass sleeve. A first stainless steel sleeve receives and retains the first glass sleeve and the isolation assembly together while a second stainless sleeve is fit over the second collimator. Apertures are defined in the first stainless steel sleeve. An end of the second stainless steel sleeve is sized to snugly fit into the end of the first stainless steel sleeve and thus properly aligns the collimators with each other. A portion of the second stainless steel sleeve underlaps the apertures of the first stainless sleeve. Welding is performed through the apertures to permanently secure the stainless steel sleeves together.